Novel applications of nanotechnology in medicine and green energy

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The development of techniques for colloidal nanoparticle synthesis has allowed scientists to fabricate materials that can manipulate light on a scale that is small even compared to the wavelength of the light itself. This ability has led to the development of myriad and diverse applications of nanostructures in wide-ranging fields. This thesis focuses on the investigation and exploitation of nanoscale material properties in the fields of medicine and energy. The unique optical properties of nanoparticles arise from their size and their high surface area to volume ratios compared to bulk materials. As a result of this relationship, the surface characteristics of nanoparticles generally dominate their properties, whereas in bulk materials the surface atoms have very little bearing on the properties of the composite. Chapter 1 gives an introduction to nanoparticles and their optical properties, including a discussion of the plasmon resonance and the properties imbued upon nanoparticles possesing such a resonance as well as the applicability of these properties that will be explored in the subsequent chapters. Chapter 2 presents a study of the interaction of cationic, hydrophobic gold nanoparticles as probes to elucidate specific regions of interest on cell surfaces. The high imaging contrast of gold nanoparticles in electron microscopy allows for visual, macroscopic observation of the aggregation patterns formed by these nanoparticles on cell surfaces. Plasmon resonant coupling between proximal nanoparticles is exploited in order to monitor nanoprobe binding and localization over time with the use of extinction spectroscopy. The role of surface proteins in the nanoparticle-cell surface interaction is elucidated, generating composite data with relevance in pharmaceutical development and pharmacokinetics. Additionally, bacteria strain-dependent toxicity is observed and subsequently investigated for smaller gold nanoparticle probes, demonstrating a potential use for nanoparticles as strain-specific antibiotics. The development of affordable, effective antibiotic technology is one of the major scientific challenges of our time; infections from pathogen-infested drinking water alone account for millions of deaths each year worldwide. In Chapter 3, we investigate the use of titanium dioxide as an inexpensive method to harness solar energy to split water into reactive species and thereby decontamitate solutions of E. coli. Though titanium dioxide is an excellent catalyst for water splitting, it requires UV irradiation, which is fairly lacking in the solar emission spectrum. Further, recuperation of titanium dioxide nanoparticles from solution is non-trivial, and its immobilization into a film greatly limits its surface area and charge carrier efficiency, thereby limiting its activity. We treat both the poor visible light absorption capability as well as the surface area limitation in this study. CdS semiconductor nanocrystals are used to extend the absorption edge of TiO₂ further into the visible light region of the spectrum by providing for lower-energy photon absorption and charge injection into titanium dioxide. TiO₂ is also electrochemically anodized to generate TiO₂ nanotube arrays, which have greatly increased surface area as well as more efficient charge transfer properties compared to thin films of TiO₂ nanoparticles. The utility of nanoparticles in increasing the light absorption of other systems continues as a theme in the work presented in the next two chapters. Chapter 4 ex- amines the plasmonic enhancement of the solar energy conversion in a biomimetic system. In this endeavor, we enhance the photocurrent generated by a light-transducing, proton-pumping protein, bacteriorhodopsin, in a 3-dimensional wet electrochemical cell. First, we increase the overall charge carrier separation with the use of a proton- selective membrane in order to minimize ionic depolarization in the cell. We then use plasmonic nanoparticles to exploit an irregularity in the bacteriorhodopsin photocycle known as the blue light effect. This effect shortens the timescale of the photocyle by more than 99% via blue photon absorption, but it has a very low natural occurrence. Plasmonic nanoparticles tuned to the blue wavelength region increase the flux of blue photons on a local level and thereby increase the overall photocurrent generation. We first examine the importance of nanoparticle field strength to photocurrent enhancement using silver nanospheres with different capping shell thicknesses. We then consider the trade-off between (1) using a nanoparticle with a plasmon resonance tuned perfectly to the blue wavelength region and (2) using a nanoparticle with a stronger field intensity but weaker energetic presence in the blue. By minimizing ionic depolarization, minimizing shielding of the plasmon electromagnetic field, and maximizing the field strength while maintaining the plasmon frequency at the proper wavelength, we demonstrate an enhancement of 5,000-fold in the photocurrent production by bacteriorhodopsin. Chapter 5 explores a variation on the theme of Chapter 4 with an application in cancer therapeutics. Here, a photodynamic cancer drug, protoporphyrin IX (PpIX), is incorporated into complexes with silver nanospheres, gold nanospheres, and gold nanorods. Each of these nanoparticles displays a plasmon resonance in a different region of the spectrum, with consequent different overlap with the absorption or emission of the drug. Photodynamic therapeutic potential is measured in situ and in vivo, and the drug activity is shown to be strongest when drug absorption overlaps with plasmon resonance. Absorption by electronic excitations in the particle crystal lattice is shown to function as a competitive light filter and decrease drug activity. Additionally, the method of attachment of the drug to the nanoparticle is examined. Maximum enhancement of drug activity is shown to require the drug to remain bound close to the nanoparticle surface, where the electromagnetic field strength is highest. This plasmonic enhancement effect on drug activity is shown to outstrip the increase in drug activity seen when using the nanoparticle solely as a delivery platform. In Chapter 6, some synthetic techniques are presented for various nanomaterials. Included are syntheses for gold, silver, and semiconductor nanoparticles of a variety of shapes and sizes as well as for TiO₂ nanotube arrays. The relationship of the ratio of capping agent to metal salt is explored for gold nanospheres, and a method for facile tuning of the longitudinal plasmon resonance displayed by gold nanorods is presented. Synthetic techniques are also presented for the nanoparticles whose applications are explored in the preceding chapters.